1. Field of the Invention
This invention relates to non-aqueous electrolytes comprising aluminium compounds. The invention also relates to cells and an electrodeposition method using the non-aqueous electrolyte.
2. Description of the Related Art
It is considered that use of aluminium as an anode material of cells leads to fabrication of cells with a high energy density at low costs. Accordingly, cells using an Al anode have been accepted as promising in the future. This is because the theoretical energy density per unit volume of Al is as high as 8050 Ah/l which is about four times larger than that of lithium. In addition, the standard electrode potential of an Al relative to a standard hydrogen electrode is -1.66 V, so that if Al is used in combination with an appropriate cathode material, the resultant cell may be interchangeable with existing alkaline dry cells or silver cells. In this sense, cells using Al as an anode are full of promise, for which developments of such cells have been extensively made. For this purpose, several problems have to be solved including those problems on the selection of liquid electrolyte, the selection of electrode material, and how to arrange a cell using an Al anode. Of these, it is the most important how to select or formulate electrolyte.
As is well known in the art, Al has been made according to an alumina electrolitic refining which requires a complicated operational procedure and a vast of electric power. Accordingly, there is a demand for the electrodeposition of Al by a simple manner. In this case, the selection of a liquid electrolyte is important.
In general, aluminium is more unlikely to be thermodynamically reduced than hydrogen, so that any electrochemical reversible reaction cannot be expected in aqueous solution systems. In addition, aluminium has an insulating and high-packed passive state natural oxide layer on the surface due to high affinity for oxygen atoms. This makes it very difficult to cause aluminium to be dissolved out at the time of discharge. As a consequence, polarity becomes great, or it will be assumed that the passive state layer is more grown through anodization.
Under these circumstances, electrolytes for primary or secondary batteries or cells making use of Al or electrolytes used for electrodeposition of Al have been proposed including, for example, organic solvent-based non-aqueous electrolytes such as used in lithium electrochemical cells, and ether-base or molten salt-based non-aqueous electrolytes. In recent years, there has also been proposed use of non-aqueous electrolytes which comprise room temperature-molten salts composed of aluminium halides/N-alkylpyridinium halides, or room temperature-molten salts composed of aluminium halides/N-alkylimidazolinium halides.
In general, however, non-aqueous electrolytes have the problem that their conductivity are lower by one or two orders of magnitude than that of aqueous electrolytes. For instance, where cells are fabricated using organic solvent-based non-aqueous electrolytes as used in lithium electrochemical cells, there arises the problem that because of the low conductivity of the electrolyte, the load characteristics of the resultant cell are lowered. In addition to the problem on the conductivity, with the ether non-aqueous electrolytes, there is a problem on handling because of the ease in firing of ethers. With non-aqueous electrolytes comprising molten salts, temperatures higher than 200.degree. C. are needed for working operations, thus presenting the problem that it is not possible to work the cell at normal temperatures. With non-aqueous electrolytes comprising room temperature-molten salts, the workable range is so narrow that once the cell has been used outside the workable temperature range, the electrolyte may be solidified or the kind or concentration of ions in the electrolyte may be changed, with a serious problem on stability.